1. Field of the Invention
The present invention relates generally to power supplies and, more specifically, the present invention relates to a switching regulator.
2. Background Information
Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency and good output regulation to power many of today""s electronic devices. In a known switched mode power supply, a low frequency (e.g. 50 or 60 Hz mains frequency), high voltage alternating current (AC) is converted to high frequency (e.g. 30 to 300 kHz) AC, using a switched mode power supply control circuit. This high frequency, high voltage AC is applied to a transformer to transform the voltage, usually to a lower voltage, and to provide safety isolation. The output of the transformer is rectified to provide a regulated DC output, which may be used to power an electronic device. The switched mode power supply control circuit usually provides output regulation by sensing the output and controlling it in a closed loop.
A switched mode power supply may include an integrated circuit switching regulator, which may include a power switch or transistor coupled in series with a primary winding of the transformer. Energy is transferred to a secondary winding of the transformer by turning on and off of the power transistor in a manner controlled by the switching regulator to provide a clean and steady source of power at the DC output. In a known switching regulator, a feedback current is sampled from the output of the DC output of the power supply. When the feedback current is below a regulation threshold, the power switch is switched at a constant frequency. However, when the feedback current is above a regulation threshold, the switching regulator is disabled, resulting in a skipped cycle of the power switch.
When cycles are skipped by a switching regulator as described above, the resulting frequency of operation of the switching regulator is reduced. Thus, the frequency of operation of the switching regulator is varied as cycles are skipped to regulate the DC output of the power supply, with the frequency decreasing as the load coupled to the DC output decreases. Generally, when the frequency of operation of known power supplies of this type drop to frequencies within the audio frequency range, such as within 20 Hz to 20 kHz, undesirable audio noise is generated by the transformers of the power supplies.
Switching regulator methods and apparatuses are disclosed. In one embodiment, a switching regulator includes a power switch coupled between first and second terminals. The first terminal is coupled to an energy transfer element of a power supply and the second terminal to be coupled to a supply rail of the power supply. A drive signal generator circuit is coupled to a third terminal to receive a feedback signal representative of an output of the power supply. The drive signal generator generates a drive signal coupled to control switching of the power switch in response to the feedback signal. The drive signal generator circuit selectively disables each on period of the drive signal in response to the feedback signal to regulate the output of the power supply. A current limit circuit is coupled to the power switch and the drive signal generator circuit to control the drive signal to limit a current flow through the power switch. The current limit circuit includes a plurality of current limit settings for the power switch that are selected in response to the feedback signal. Additional features and benefits of the present invention will become apparent from the detailed description, figures and claims set forth below.